Trifluoromethanesulfonic acid, catalysis

As an alternative to lithium enolates. silyl enolates or ketene acetals may be used in a complementary route to pentanedioates. The reaction requires Lewis acid catalysis, for example aluminum trifluoromethanesulfonate (modest diastereoselectivity with unsaturated esters)72 74 antimony(V) chloride/tin(II) trifluoromethanesulfonate (predominant formation of anti-adducts with the more reactive a,/5-unsaturated thioesters)75 montmorillonite clay (modest to good yields but poor diastereoselectivity with unsaturated esters)76 or high pressure77. 

In principle, sulfonyl compounds bearing highly-electron-accepting substituents are able to transfer the sulfonyl group as an electrophile. Thus, the exchange of aryl substituents in methyl aryl sulfones under catalysis of trifluoromethanesulfonic acid takes place258 (equation 46). This reaction represents a further example for the reversibility of Friedel-Crafts reactions. 

Addition reactions such as A-alkylation do not occur readily, and trimethylsilylmethylation of 3,4-diphenyl-l,2,5-thiadiazole 8 with trimethylsilylmethyl trifluoromethanesulfonate at 80°C occurred at N-2 < 1999J(P1) 1709>. The electron-rich 3-hydroxy-l,2,5-thiadiazole can be preferentially methylated on N-2 using trimethyl orthoacetate in toluene to afford the 2-methyl-l,2,5-thiadiazol-3-one in 69% yield <2002EJ01763>, although a mixture of 3-hydroxythiadiazole and neat trimethyl orthoacetate showed a 20 80 ratio of N- versus 0-alkylation products by H NMR. Treatment of 3-hydroxy-l,2,5-thiadiazole with /-butyl acetate under acid catalysis (Amberlyst 15) gave almost exclusively the A-alkylated compound <2002BMC2259>. 

In the course of our investigations to circumvent the second drawback in the use of water (the decomposition problem), we have found that some metal salts such as rare earth metal triflates (triflate = trifluoromethanesulfonate) can be used as water-compatible Lewis acids [16,17]. Lewis acid catalysis has attracted much attention in organic synthesis [18]. Although various kinds of Lewis acids have been developed and many have been applied in industry, these Lewis acids must be generally used under strictly anhydrous conditions. The presence of even a small amount of water stops the reactions, because most Lewis acids immediately react with water rather than substrates. In addition, recovery and reuse of the conventional Lewis acids are formidable tasks. These disadvantages have restricted the use of Lewis acids in organic synthesis. 

As homogeneous acid catalysts, trifluoromethanesulfonic acid (CF3SO3H) and its silylated form, trimethylsilyl triflate (CF3S03SiMc3), were chosen for comparison of the acid catalysis between heterogeneous and homogeneous acids. The reaction of 14a with 15a or 15b proceeded smoothly even at low temperature in the presence of CF3S03SiMe3 as well as CF3SO3H (Table XIV, Entries 8, 10 Table XV, Entries 12, 13, 15). The catalytic behavior of... 

A variant of the preceding pathway has been described in which the methyl ester of (275) on treatment with a small amount of cupric trifluoromethanesulfonate in methylene chloride under oxygen for several hours affords (9) <90CC45i>. The yields of (9) are similar to those previously obtained by acid catalysis which suggests that adventitious acid, rather than cupric salt, may have been responsible for the result. 

The synthesis has been described by Karrer et al. (1925). This product has also been prepared in one step from 2-pentanone and acetonitrile using copper(II) trifluoromethanesulfonate under acid catalysis (Nagayoshi and Sato, 1983). 

As mentioned earlier, in catalyses by elemental iodine or perfluorinated iodoaUcanes [86-88, 124] it is sometimes difficult to rule out that traces of acid contribute at least partially to the observed reactivity. Thus, in this study our goal was to show as unambiguously as possible that halogen bonding is indeed responsible for the activation of the substrate. In the solvolysis reaction of benzhydryl bromide, accidental acid catalysis by impurities can safely be ruled out as the use of even 1 equiv. of the strong acid HOTf (trifluoromethanesulfonic acid) yields only 25% of 22 under otherwise identical conditions. Furthermore, trace amounts of acid can be quenched by the addition of 10 mol% of pyridine to the reaction, whereas the effect of the bis(iodoimidazohum) activators is only marginally affected by this additional component. 

Further studies carried out on methyl artemisinate (278) confirm that the hydroperoxide (279) rearranges to the enol (280) which is subsequently autoxidized to (281) and thence to artemisitene (282) under catalysis with acid or copper trifluoromethanesulfonate (Scheme 41) <95JA11098>. It is likely that (279) rearranges by homolysis to the 1,4-diradical (283) which then cleaves to (280). In... 

MMTS MsOH NBS NHMDS NMP NMR PPb Ph Pr PTC rt TBDMS Tf THF THP TLC TMEDA TMS TMSOTf Tol TOMAC Ts TsOH UDP methyl methylthiomethyl sulfoxide (=FAMSO) methanesulfonic acid N-bromosuccinimide sodium hexamethyldisililazide /V-methyl-2-pyrrolidone nuclear magnetic resonance parts per billion phenyl propyl Phase transfer catalysis room temperature t-butyldimethylsilyl triflatc (trifluoromethanesulfonate) tetrahydrofuran 2-tetrahydro-2//-pyran-2-yl thin-layer chromatography /V./V./V /V -tetramethylethylenediamine trimethylsilyl trimethylsilyl triflate p-tolyl trioctylmethylammonium chloride tosyl p-toluenesulfonic acid ultrasonically dispersed potassium... 

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